Adiponectin is an insulin-sensitive hormone which is specifically secreted from fat cells, and is present in the blood at a relatively high level (5 to 10 μg/mL). A physiological effect of adiponectin has previously been reported by the present inventors; i.e., decrease in adiponectin level is a cause for type-II diabetes after obesity or a cause for sugar or lipid metabolism disorder in lipoatrophic diabetes (Non-Patent Document 1). Medical applications of adiponectin have been proposed, including diagnosis of sugar metabolism disorder; in particular, monitoring of therapeutic effect of a thiazolidine derivative which is an insulin-resistance-improving drug (Patent Document 1), and dosage of hepatic fibrogenetic suppressor utilizing effect of adiponectin, including suppression of activation and growth of stellate cells and suppression of production of extracellular matrix (Patent Document 2).
Adiponectin is classified into the C1q (Complement 1q) family on the basis of its structure, and thus has a collagen-like domain, which is a characteristic feature of the C1q family. Adiponectin has been reported to form trimer-unit-based multimers (Non-Patent Documents 2 and 3).
Relationship between the structures of adiponectin and physiological effect, activity, etc. has been reported. Tsao et al. have reported that effect of activating NF-κB is observed only in adiponectin multimers which are larger than a trimer but is not observed in a trimer and in a globular domain which lacks the collagen-like domain. Utpal et al. have reported that only high-molecular-weight adiponectin is significantly decreased through administration of insulin or glucose, and that adiponectin in which cysteine in the collagen-like domain is substituted by serine or adiponectin which has undergone a reducing treatment exhibits strong blood-sugar-reducing effect, but substantially no blood-sugar-reducing effect is observed in adiponectin which has undergone no reducing treatment and in globular adiponectin (Non-Patent Document 4).
However, adiponectin employed in these reported studies are produced through gene recombination by use of E. coli or the like. Therefore, behavior or action of adiponectin that is produced in an organism may not be reflected accurately. In addition, blood adiponectin level is known to differ between sexes. As compared with males, females have been reported to have significantly high blood adiponectin level (Non-Patent Document 5). A test employing mouse serum has revealed that females are high particularly in high-molecular-weight adiponectin content (Non-Patent Document 4).
As described above, through measurement of only the total amount of adiponectin, sufficient information cannot be obtained about the relationship between the structures of adiponectin and physiological effects or diseases. Therefore, there has still been demand for a method for selectively assaying adiponectin multimers present in a biological sample.
In a prior art method for assaying adiponectin, a sample to be measured is boiled in the presence of sodium dodecylsulfate (SDS) before immunoassay is performed (Patent Document 3). The boiling treatment enables exposure of an antibody-recognizing site of adiponectin which has been hidden stereostructually, in order to immunologically determine the total amount of adiponectin. Therefore, this method is not directed to selectively assay of adiponectin multimers.
A method for assaying adiponectin contained in a sample with neither SDS denaturing treatment nor thermal denaturing treatment has been disclosed (Patent Document 2, It is described in the document that native adiponectin was measured). However, no reference has been made to selectively assay, and this method cannot be employed for selectively assay.    [Patent Document 1] WO 03/016906    [Patent Document 2] JP-A-2002-363094    [Patent Document 3] JP-A-2000-304748    [Non-Patent Document 1] Yamauchi T., et al., Nat. Med., 7, 941-946, 2001    [Non-Patent Document 2] Nakano Y., et al., J. Biochem., 120, 803-812, 1996    [Non-Patent Document 3] Tsao T-S., et al., J. Biol. Chem., 277, 29359-29362, 2002    [Non-Patent Document 4] Utpal B., et al., J. Biol. Chem., 278, 9073-9085, 2003    [Non-Patent Document 5] Yamamoto Y., et al., Clin. Sci., 103, 137-142, 2002